1. Field of the Invention
This invention relates to the field of flow control in a compressible fluid. More specifically, the invention comprises a piezoelectric actuator for varying the throat geometry of a microjet nozzle, thereby varying the characteristics of the microjet produced by the microjet nozzle.
2. Description of the Related Art
Microjet nozzles are reduced-scale conventional nozzles configured to produce small jets of fast moving compressible fluid (“microjets”). Air is the most commonly used fluid. Microjets can be employed to modify air flowing over a surface and have many applications, such as modifying flow over aircraft lift and control surfaces.
Microjets can be subsonic, transonic, or supersonic. FIG. 1 shows a simple nozzle configured to create a supersonic microjet. Microjet nozzle 10 is a passage bounded by a revolved wall profile. While microjets need not always have a circular cross section, this is generally the cross section employed. In order to create a supersonic microjet, incoming flow 12 is directed through converging section 14, throat 22, and diverging section 16. Those skilled in the art will know that the converging section accelerates the flow in its subsonic phase. The flow will be transonic through the region of throat 22. The flow will then become supersonic and accelerate further as it passes through diverging section 16. Microjet 20 is then formed as the flow passes out of nozzle exit 18.
The throat diameter of a microjet nozzle will typically lie between about 100 micrometers and about 1000 micrometers. The pressure ratio between the microjet produced and the surrounding atmosphere will typically lie between about 1.01 and about 50. Some examples may lie outside these ranges.
It is known to vary the characteristics of the microjet by varying the input pressure. This will alter the velocity of the gas flowing through the microjet nozzle. Unfortunately, though, varying the input pressure will not rapidly alter the flow. One of the applications for microjets is active boundary layer control. This application would benefit from the ability to rapidly alter the microjet's characteristics. A pressure variation frequency of 100 Hz to 10 kHz would be beneficial in many applications. There are presently pulsed mass flow actuators that are capable of achieving frequencies in the low kHz regime. However, these actuators are limited to a narrow range of frequencies. It is preferable to provide a microjet nozzle/actuator combination which can achieve high cyclic rates while retaining the ability to operate across a wide range of frequencies. The present invention proposes an actuator which can produce such rapid cycling.